Solid state light sources may be used to provide colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. White solid state emitters have been investigated as potential replacements for white incandescent or fluorescent lamps for increased efficiency and longevity. Longevity of solid state emitters is of particular benefit in environments where access is difficult and/or where change-out costs are extremely high. Despite potential efficiency gains, solid state light sources may present challenges in simultaneously achieving high efficacy, good color reproduction, and color stability with respect to variations in operating temperature.
The terms “white light” or “whiteness” do not clearly cover the full range of colors along the Planckian or blackbody locus (BBL) since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of correlated color temperature (CCT) and proximity to the BBL. The white area proximate to (i.e., within approximately eight MacAdam ellipses of) the BBL and having a correlated color temperature between 2,500 K and 10,000 K, is shown in FIG. 1 (based on the 1931 CIE diagram).
As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. Thus, apparent colors of incandescing materials are directly related to their actual temperature (in Kelvin (K)). Practical materials that incandesce are said to have CCT values that are directly related to color temperatures of blackbody sources. CCT is intended to characterize the apparent “tint” of the illumination (e.g., warm or cool) produced by an electric light source.
General illumination generally has a CCT between 2,000-10,000 K, with the majority of lighting devices for general illumination having CCT values in a range of from 2,700-6,500 K. Quality artificial lighting generally attempts to emulate the characteristics of natural light. Natural light sources include daylight with a relatively high color temperature (e.g., ˜5,000 K) and incandescent lamps having a lower color temperature (e.g., ˜2,800 K).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). CRI and CRI Ra provide relative measures of the shift in surface color and brightness of an object when lit by a particular lamp, wherein CRI utilizes fourteen indices (reference colors), and CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. Daylight has a high CRI (CRI Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and with fluorescent lighting being less accurate (typical Ra of 70-80) and suitable for general illumination in contexts where accurate color perception is less important. For some general interior illumination, a CRI Ra>80 is acceptable. Sources with CRI Ra>85, and more preferably, CRI Ra>90, provide greater color quality.
CRI Ra (or CRI) alone is not a satisfactory measure of the benefit of a light source, since it confers little ability to predict color discrimination (i.e., to perceive subtle difference in hue) or color preference. There appears to be a natural human attraction to brighter color. Daylight provides a spectrum of light that allows the human eye to perceive bright and vivid colors, which allows objects to be distinguished even with subtle color shade differences. Accordingly, it is generally recognized that daylight is the “best” light for emphasizing and distinguishing color. The ability of human vision to differentiate color is different under CCT conditions providing the same CRI Ra. Such differentiation is proportional to the gamut of the illuminating light.
Gamut area of a light source can be calculated as the area enclosed within a polygon defined by the chromaticities in CIE 1976 u′v′ color space of the eight color chips used to calculate CRI Ra when illuminated by a test light source. Gamut area index (GAI) is a convenient way of characterizing in chromaticity space how saturated the illumination makes objects appear—with a larger GAI making object colors appear more saturated. GAI is a relative number whereby an imaginary equal-energy spectrum (wherein radiant power is equal at all wavelengths) is scored as 100. GAI for a test source is determined by comparing color space area of the light being tested to the color space area produced by the imaginary or theoretical equal-energy spectrum (EES) source. Unlike CRI Ra (or CRI), which has a maximum value of 100, GAI can exceed 100, meaning that some sources saturate colors more than an equal-energy source serves to saturate color.
It is found that typical blackbody-like light sources and typical daylight-like light sources have different gamut areas. Low CCT sources (e.g., incandescent emitters) have a gamut area index of approximately 50% (i.e., about half the gamut area of the EES source). Sources with higher CCT values have a larger GAI. For example, a very bluish light with a CCT of 10,000K may have a GAI of 140%.
Wavelengths of light outside the visible spectrum have limited utility for direct illumination because they cannot be seen by the human eye, although it is noted that short wavelength non-visible light can cause certain materials to fluoresce. Even within the visible spectrum, the human eye exhibits greater response to some wavelengths of light than to others. Response of the human eye to light also varies with respect to the level of intensity of light.
At the back of a human eye, the retina contains millions of light receptors that convert light into electrified signals that are sent to vision centers of the brain. The two major categories of photoreceptors are called cones and rods because of their geometric shapes. The central part of the retina, called the fovea, contains only cones. The rest of the retina is populated with both rods and cones, with the number of rods exceeding the number of cones by a ratio of about 10 to 1.
Photopic vision is the vision of the eye under well-lit conditions. In humans and many other animals, photopic vision is mediated by cone cells, and allows color perception and significantly higher visual acuity than available with scotopic vision. A human eye uses three types of cones with biological pigments having maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green), and 564 nm (yellowish green) to sense light in three bands of color. Such cones provide maximum efficacy of about 683 lm/W at a wavelength of 555 nm (green).
Scotopic vision is the vision of the eye under low light conditions. In a human eye, cone cells are non-functional in low light; as a result, scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of light around 498 nm (green-blue) and are insensitive to wavelengths longer than about 640 nm (red). For young eyes, scotopic vision may peak around 507 nm, with a sensitivity equivalent to about 1700 lm/W. Scotopic vision results in poor color discrimination. Scotopic vision occurs at luminance levels of 10−2 to 10−6 cd/m2; photopic vision occurs at luminance levels of 1 to 106 cd/m2 (normal light); and mesoptic vision occurs in intermediate lighting conditions (luminance levels of 10−2 to 1 cd/m2). Mesoptic vision is effectively a combination of scotopic and photopic vision, but yields inaccurate visual acuity and color discrimination.
FIG. 2 illustrates scotopic and photopic luminosity functions, with the leftmost curve embodying a scotopic luminosity function (as adopted by the Commission Internationale de l'Éclairage (CIE) in 1951), and with the rightmost curve embodying photopic luminosity functions (wherein the solid line represents the CIE 1931 standard, the dashed curve represents the Judd-Vos 1978 modified data, and the dotted curve represents the Sharpe, Stockman, Jagla & Jägle 2005 data). The CIE 1931 photopic luminosity function also forms the central color matching function in the CIE 1931 color space. As shown in FIG. 2, the scotopic curve exhibits significant response above 420 nm, a peak at 507 nm, and very little response above 600 nm, whereas the photopic curve exhibits very limited response below 450 nm, a peak at 555 nm, and significant response above 650 nm before declining to zero response around 700 nm. For everyday light levels, the photopic luminosity function best approximates the response of the human eye; however, for low light levels, the response of the human eye changes and the scotopic luminosity function applies. This difference in the scotopic and photopic luminous efficacy of the source results from the shift of eye sensitivity function from photopic to scotopic conditions (i.e., peaking at 555 nm under photopic lighting conditions, while peaking at 507 nm under scotopic lighting conditions), also known as the Purkinje shift.
Historically, lighting manufacturers have utilized light meters to determine lumen output of a lamp, with such light meters being calibrated by examining the eye's sensitivity to only cone-activated vision in the fovea (i.e., the central part of the retina), while ignoring the effect of rod-activated vision. As a result, traditional lighting practice accepted a single sensitivity function based on the assumption that the more light-sensitive rods only functioned at very dim light levels. More recent studies have demonstrated that rod photoreceptors are active not only in dim light but also at typical interior light levels as well.
Since rods are more sensitive than cones to bluish-white light sources characteristic of higher CCT values, light sources with higher S/P ratios may potentially provide equivalent levels of perceived brightness and visual acuity at lower output power levels. This explains why environments lit by warm white (3000K) and even cool white (4100K) lamps (e.g., fluorescent lights) appear less bright than the same environment lit by lamps of a higher CCT values, such as 5000K or above. Light with high S/P ratios (typically correlated with higher CCT values) yields relatively smaller pupils at a given photopic light level, so that object light rays are collected more at the central region of the eye, which may result in improved optical vision. Since an environment may be illuminated with lamps having higher S/P ratios at lower power levels but provide perceived brightness levels equivalent to those attainable with lamps having lower S/P ratios at higher power levels, use of lamps having higher S/P ratios may provide basis for saving energy.
It may be challenging to provide solid state lamps with elevated S/P ratios in combination with reasonably high gamut area index values, particularly in desirable CCT ranges (e.g., from 2700K to 5000K).
The art continues to seek improved lighting devices that address one or more limitations inherent to conventional devices.